Anatomy fill custom root

[iLLiCiTiT]

Issue

1.) Anatomy does not allow to fill different roots than has defined in roots.json, which is rule by design of anatomy, but it may be helpfull to fill rootless path with custom root.
2.) Another issue is that find_root_template_from_path does not check if root value is filled with nonempty string so may return invalid values.

Changes

1.) Implemented fill_root_with_path class method which fill root key in rootless path with entered directory path, no matter if is single or multiroot. Single and multiroot is only reason why this helper method is required.
2.) find_root_template_from_path check root values if are empty.

[jakubjezek001]

works well